This invention relates to a coating composition which has excellent electrical insulating properties, which can be cured at lower than normal temperatures, and which provides reduced die wear in punching operations, e.g. of electrical steel sheet having such a coating thereon.
Two principal types of coatings are currently used for insulation of electrical steel sheet product, namely, those based on aluminum phosphate and certain silicate resin compositions. The phosphate systems have a tendency to become powdery during punching operations in low temperature, low humidity environments and to absorb moisture and become sticky at higher temperatures in high humidity environments. The silicate-resin systems are less corrosion resistant and several times more expensive than the phosphate systems. Thus, both systems have significant disadvantages and a coating overcoming these disadvantages is needed.
An insulating coating for electrical sheet containing sodium silicate, carbohydrate (such as sugar) and various insulating or filler materials such as talc, silica, clays, mica, magnesia mica, and insulating oxides such as aluminum oxide, iron oxide and magnesium oxide is disclosed in U.S. Pat. No. 1,750,305. Also, this coating is baked at temperatures of 120.degree. to 150.degree. C. A coating for the same purpose containing about 45% aluminum oxide and 15% sodium silicate is disclosed in U.S. Pat. No. 1,924,311. This reference also discloses that iron or calcium oxide, or highly calcined or fused magnesium oxide, or a mixture of two or more of such substances may be substituted for all or a portion of the aluminum oxide. The use of borax or borates as a binding agent is also disclosed. Another coating for electrical sheet is disclosed in U.S. Pat. No. 1,982,179. This coating contains lime, iron oxide and water glass. The reference also suggests that magnesium oxide may be substituted for the lime or both may be used. Finally, an insulative coating containing magnesium oxide and 2 to 5% bentonite (defined as a natural colloidal crystalline inorganic hydrous aluminum silicate) for electrical sheet is disclosed in U.S. Pat. No. 2,739,085.
It is a primary object of this invention to provide a coating which does not become powdery or sticky, has good corrosion resistance and electrical insulation properties before and after quality development annealing, can be cured at relatively low temperatures and provides reduced die wear during punching operations, e.g. of electrical steel sheet having the coating thereon.